The structure of an ATP-bound kinesin motor domain has been predicted and conformational differences relative to the known ADP-bound form of the protein were identified,* as reported in reference [96]. The differences should be attributed to force-producing ATP hydrolysis. Candidate ATP-kinesin structures were obtained by simulated annealing, by placement of the ATP gamma-phosphate in the crystal structure of ADP-kinesin, and by inter-atomic distance constraints. The choice of such constraints was based on mutagenesis experiments, which identified Gly234 as one of the gamma-phosphate sensing residues, as well as on structural comparison of kinesin with the homologous ncd motor and with G proteins. The prediction of nucleotide-dependent conformational differences reveals an allosteric coupling between the nucleotide pocket and the microtubule binding site of kinesin. Interactions of ATP with Gly234 and Ser202 trigger structural changes in the motor domain, the nucleotide acting as an allosteric modifier of kinesin's microtubule-binding state. We suggest that in the presence of ATP kinesin's putative microtubule binding regions (L8, L12, L11, alpha4, alpha5 and alpha6) form a face complementary in shape to the microtubule surface. In the presence of ADP, the microtubule binding face adopts a more convex shape relative to the ATP-bound form, reducing kinesin's affinity to the microtubule.